A large number of objects are currently produced from thermoplastic materials, from toys to various types of vehicle components including those for aircraft and space craft, for example. While some of these objects require only a single layer of the plastic, many (particularly those of vehicles) involve multiple layers so as to form a "composite". Often these layers of plastic will include a strengthening material in the form of reinforcing fibers. In the field of spacecraft part fabrication, the plastic is frequently in the form of thin layers of a polyether ether ketone (PEEK) containing carbon fibers, where the fibers are present at about 60% by volume. These layers, referred to as a "prepreg", are typically formed using a tape about 0.005 in. thick, and commercially-available tapes of this material having a width up to about thirty-six inches are available from several sources, including Amoco Chemical company.
It is common prior art practice for the forming of the composites to place one or more layers of the thermoplastic sheet over a die of the desired configuration. This assembly is placed in a bag, which is evacuated, and then in an autoclave where it is heated (typically 700.degree.-800.degree. F.) under pressure (typically 100-200 psi) for a time selected for a particular application. Under these conditions the thermoplastic conforms to the shape of the die. The composite can then be cooled and removed from the autoclave. There is essentially no restriction on the contour of the part.
It can be seen, however, that this forming method is limited to parts having a size and shape that can be processed within an autoclave. When larger components are to be produced, or when a repair to a previously-prepared composite is to be made, other methods are required.
Several devices have been developed to achieve this non-autoclave compaction of thermoplastic tape. The simplest form of a device of this character is a roller that subjects the elements for the consolidation to pressure immediately after the application of heat thereto. A modification of this is the compliant roller described in U.S. Pat. No. 5,078,821 issued on Jan. 7, 1992. This device, while applying a line of pressure, fails to provide the pressure over a significant area.
Other devices conceived for this consolidation include a unit employing a plurality of spring-loaded rollers that are part of a continuous chain. These rollers, again, provide pressure along a given line and have no flexibility across their own length. The same is true of a "silent chain" that is similar to a interior toothed belt that does not move relative to the composite structure. Still another device is a "stomper" wherein individual spring-loaded feet apply pressure. Since this latter device has to be lifted from the surface in order to achieve relative motion with respect to the object, unequal pressures can be applied that result in regions of insufficient consolidation.
Accordingly, it is an object of the present invention to provide a device that achieves sufficient flow of thermoplastic materials during the heating thereof such that full consolidation of multiple layers is achieved.
It is another object of the present invention to provide a device that moves relative to an object to be consolidated, with that device providing pressure over a significant area but with sufficient flexibility so as to form components having curved contours.
These and other objects of the present invention will become apparent upon a consideration of the drawings that follow together with a complete description thereof.